1. Field of the Invention
The present invention relates to contact displacement meters, in particular, to an optical absolute type contact displacement meter for reading a relative displacement of a built-in scale with a line sensor.
2. Description of the Related Art
The contact displacement meter includes a contact attached so as to be movable with respect to a housing, and measures the displacement of a measuring object by contacting the contact to the measuring object and measuring the relative displacement of the contact with respect to the housing with a detector incorporated in the housing. The detector incorporated in the housing may be configured by a magnet and a differential transformer, may be configured by a scale formed with a predetermined pattern and an electrostatic capacity, magnetic, optical sensor or the like for reading the scale.
Among these contact displacement meters, the contact displacement meter for reading the pattern formed on the scale using light can realize a high precision displacement meter in a relatively easy manner by enhancing the mechanical formation accuracy of the pattern.
However, demand for miniaturization is strong as the contact displacement meter is often used by being attached to a device, and the optical detector incorporated in the housing is generally configured as a so-called incremental type for counting the scale pattern due to restrictions in size of the light emitting element, the light receiving element, and the like.
As an optical absolute position length measuring device for reading the relative displacement of the scale with a line sensor, for example, Japanese Unexamined Patent Publication No. 2000-241115 discloses a length measuring device capable of obtaining an absolute position by irradiating light emitted by a light emitter onto a transition lattice part including an absolute position pattern that provides absolute position information and a precision optical lattice, and analyzing the output received by a light receiving element arrayed at a predetermined interval.
The absolute position length measuring device disclosed in Japanese Unexamined Patent Publication No. 2000-241115 first specifies a coarse absolute position using the absolute position pattern. The relative position is then calculated based on the output signal by the light passed through the precision optical lattice, and the relative position is then added or subtracted to or from the coarse absolute position to obtain the fine absolute position.
The contact displacement meter incorporating the incremental optical detector can respond to the demand of miniaturization, demand of higher accuracy, and the like. However, if the contact is suddenly moved, miscounting of the scale pattern occurs, and a correct measurement result cannot be obtained.
In optically measuring the absolute position of the contact, the absolute position information of the transition lattice part that moves with the contact need to be reliably read. When calculating the absolute position by the difference in the received light signal at the light receiving element as in the absolute position length measuring device disclosed in Japanese Unexamined Patent Publication No. 2000-241115, an elaborate parallel light needs to be irradiated on the absolute position pattern and the precision optical lattice.
However, it is difficult to have the light irradiated from the light source as a precise parallel light, and furthermore, even in a case where a precise parallel light is generated, unnecessary reflection occurs at the boundary edge portion of a light shielding region and a light passing region of the pattern, leakage light (or leakage charge) to the adjacent pixel occurs at the image sensor, and the like, and thus an ideal received light waveform as disclosed in Japanese Unexamined Patent Publication No. 2000-241115 is difficult to obtain. Thus, the received light signal at the light receiving element includes various optical noises, reading of the absolute position information becomes difficult only from the received light signal of the light passed through the absolute position pattern and the precision optical lattice, and sufficient measurement accuracy cannot be ensured.
In the case of a Fourier image type as shown in Japanese Unexamined Patent Publication No. 05-346330, a diffraction phenomenon occurs with respect to the light passed through the fine absolute position pattern and the precision optical lattice, and thus an ideal received light waveform as disclosed in Japanese Unexamined Patent Publication No. 2000-241115 becomes difficult to obtain. Thus, the received light signal at the light receiving element includes various optical noises, reading of the absolute position information becomes difficult only from the received light signal of the light passed through the absolute position pattern and the precision optical lattice, and sufficient measurement accuracy cannot be ensured.